I am interested in building a audio inputs switch (audio selector). I have experience in digital circuits, but not much in analog circuits, but I am passionate about audio. So I need a little help.

Had thought of using a pair of 4051 with 8 inputs each, and also a small micro controller (atmel avr or microchip pic). It would be better to use the 4052? (I prefer to have 8 stereo inputs with two 4051, one for each channel)

Someone can recommend me any design or schematic?

I have to put a "audio buffer" or preamp? in the output? in the input? in two places? (if so, could use a NE5532?)

Any information is welcome: schematics, designs, tips, ideas... (I need help especially in the analog audio part (input or output preamplifier, buffer...)

I'm not sure about the quality of the old CMOS muxes. There's newer and more expensive muxes available from mfgrs like Maxim and Analog Devices.

ADG508FBN from Analog Devices is one that I've used in data acquisition applications. The specs are pretty good, and it has the sometimes useful feature of going high-impedance on the inputs when unpowered. The dual 4:1 version would be more useful, the ADG509F, since one chip is all you need for a stereo four-input switch.

Maxim's versions are the MAX4708 and MAX4709, if those are more available. Just beware of chips from vendors in China that are used non-faultprotected MUX chips that have been removed from boards and re-marked. I nearly got stung with 100s of those, buying through a broker.

4051 and the likes have a resistance that varies with the applied signal voltage. If you don't object to phase inverson, - using an opamp as an inverting amp- try to use the mux part inside a summing/current amp - mux output directly couples to the - input, and choose series and feedback resistors to match your impedance and gain demands.

Please bear in mind that these chips are NOT silent switching. By their very nature, there is an artifact at the time of switching. The Analog Devices purpose built mux/switches are better.
Also, put the switch at the virtual earth point of the output buffer amp so you are switching current, not volts.